In botany, a leaf is an above-ground plantorgan specialized for photosynthesis. For this purpose, a leaf is typically flat and thin, to expose the chloroplast containing cells (chlorenchyma tissue) to light over a broad area, and to allow light to penetrate fully into the tissues. Leaves are also the sites in most plants where respiration, transpiration, and guttation take place. Leaves can store food and water, and are modified in some plants for other purposes. The comparable structures of ferns are correctly referred to as fronds.

Fallen leaf of a maple. Note areas where chlorophyll (green) has been destroyed now appear yellow.

Leaf structure

A structurally complete leaf of an angiosperm consists of a petiole (leaf stem), a lamina (leaf blade), and stipules (small processes located to either side of the base of the petiole). The point at which the petiole attaches to the stem is called the leaf axil. Not every species produces leaves with all of these structural parts. In some species, paired stipules are not obvious or are absent altogether; a petiole may be absent; or the blade may not be laminar (flattened). The tremendous variety shown in leaf structure (anatomy) from species to species is presented in detail below under Leaf types, arrangements, and forms.

A leaf is considered to be a plant organ, typically consisting of the following tissues:

An epidermis that covers the upper and lower surfaces

An interior chlorenchyma called the mesophyll

An arrangement of veins (the vascular tissue).

Epidermis

The epidermis is the outer multi-layered group of cells covering the leaf. It forms the boundary between the plant and the external world. The epidermis serves several functions: protection against water loss, regulation of gas exchange, secretion of metabolic compounds, and (in some species) absorption of water. Most leraves show dorsoventral anatomy: the upper (adaxial) and lower (abaxial) surfaces have somewhat different construction and may serve different functions.

The epidemis is usually transparent (epidermal cells lack chloroplasts) and coated on the outer side with a waxy cuticle that prevents water loss. The cuticle may be thinner on the lower epidermis than on the upper epidermis; and is thicker on leaves from dry climates as compared with those from wet climates.

The epidermis tissue includes several differentiated cell types: epidermal cells, guard cells, subsidiary cells, and epidermal hairs (trichomes). The epidermal cells are the most numerous, largest, and least specialized. These are typically more elongated in the leaves of monocots than in those of dicots.

The epidermis is covered with pores called stomata (sing., stoma), part of a stoma complex consisting of a pore surrounded on each side by chloroplast-containing guard cells, and two to four subsidiary cells that lack chloroplasts. The stoma complex regulates the exchange of gases and water vapor between the outside air and the interior of the leaf. Typically, the stomata are more numerous over the abaxial (lower) epidermis than the (adaxial) upper epidermis.

Mesophyll

Most of the interior of the leaf between the upper and lower layers of epidermis is a parenchyma (ground tissue) or chlorenchyma tissue called the mesophyll (= middle leaf). This "assimilation tissue" is the primary location of photosynthesis in the plant (The products of photosynthesis are called assimilates).

In ferns and most flowering plants the mesophyll is divided into two layers:

an upper palisade layer of tightly packed, vertically elongated cells, one to two cells thick, directly beneath the adaxial epidermis. Its cells contain many more chloroplasts than the spongy layer. These long cylindrical cells are regularly arranged in one to five rows. Cylindrical cells, with the chloroplasts close to the walls of the cell, can take optimal advantage of light. The slight separation of the cells provides maximal absorption of carbon dioxide. This separation must be minimal to afford capillary action for water distribution. In order to adapt to their different environment (such as sun or shade), plants had to adapt this structure to obtain optimal result. Sun leaves have a multi-layered palisade layer, while shade leaves or older leaves closer to the soil, are single-layered.

Beneath the palisade layer is the spongy layer. The cells of the spongy layer are more rounded and not so tightly packed. There are large intercellular air spaces. These cells contain less chloroplasts than....

The pores or stomata of the epidermis open into substomatal chambers, connecting to air spaces between the spongy layer cells.

These two different layers of the mesophyll are absent in many aquatic and marsh plants. Even an epidermis and a mesophyll may be lacking. Instead for their gaseous exchanges they use a homogenous aerenchyma (thin-walled cells separated by large gas-filled spaces). Their stomata are situated at the upper surface.

Leaves are normally green in color, which comes from chlorophyll found in plastids in the chlorenchyma cells. Plants that lack chlorophyll cannot photosynthesize.

Leaves in temperate, boreal, and seasonally dry zones may be seasonally deciduous (falling off or dying for the inclement season). This mechanism to shed leaves is called abscission. After the leaf is shed, a leaf scar develops on the twig. In cold autumns they sometimes turn yellow, bright orange or red as various accessory pigments (carotenoids and anthocyanins) are revealed when the tree responds to cold and reduced sunlight by curtailing chlorophyll production.

Veins

The veins are the vascular tissue of the leaf and are located in the spongy layer of the mesophyll. They are typical examples of pattern formation through ramification.

phloem, which usually moves sap out, the latter containing the glucose produced by photosynthesis in the leaf.

The xylem typically lies over the phloem. Both are embedded in a dense parenchyma tissue (= ground tissue), called pith, with usually some structural collenchyma tissue present.

Leaf types, arrangements, and forms

The external leaf characteristics (such as shape, margin, hairs, etc.) are important for identifying plant species, and botanists have developed a rich terminology for describing leaf characteristics.

The leaves on this plant are arranged in pairs opposite one another, with successive pairs at right angles to each other ("decussate") along the red stem. Note developing buds in the axils of these leaves.

Leaves may be classified in many different ways, and the type is usually characteristic of a species, although some species produce more than one type of leaf. The terminology associated with describing leaf morphology is presented (with illustrations) at Wikibooks.

Spiral — leaf attachments arranged in interlocking spirals, with the spirals in Fibonacci number ratios (1:1:2:3:5:8:13:21:etc; e.g. five successive leaves counted along a shoot clockwise, and eight leaves anti-clockwise, come to the same leaf).

Alternate — leaf attachments singular at nodes, and alternate direction up the stem.

Opposite — leaf attachments paired at each node; decussate if, as typical, each successive pair is rotated 90 going along the stem; or distichous if not rotated, but all two-ranked, in the same plane.

Whorled — three or more leaves attach at each point or node on the stem. As with opposite leaves, successive whorls may or may not be decussate, rotated by half the angle between the leaves in the whorl (i.e., successive whorls of three rotated 60, whorls of four rotated 45, etc). Note: opposite leaves may appear whorled near the tip of the stem.

Rosulate — leaves form a rosette ( = a cluster of leaves growing in crowded circles from a common center).

Leaves of the Norway Spruce (Picea abies) are needle-shaped and the arrangement is spiral

Leaves are set in a particular arrangement around the stem in order to gain an optimal yield of light. Subsequent leaves are arranged in spirals, clockwise or counterclockwise, with always the same angle of divergence. There is a certain regularity in these angles: they follow the numbers in a Fibonacci series: 1/2, 2/3, 3/5, 5/8, 8/13, 13/21, 21/34, 34/55, 55/89. This series tends to a limit of 360 x 34/89 = 137,52 or 137 30'. This angle is known mathematically as the 'golden angle'. The numerator gives the number of gyres, till the leaf arrives at the same initial position. The denominator gives the number of leaves in this arrangement. This can easily be seen.

Divisions of the lamina (blade):

Simple leaves have an undivided blade. The leaf shape may be one of deeply divided lobes, but the gaps between lobes do not reach to the vein.

Compound leaves have divided blades, each leaflet separated along a main or secondary vein.

Palmately compound leaves have the leaflets radiating from the end of the petiole, like fingers off the palm of a hand. There is no rachis, e.g. Cannabis (hemp) and Aesculus (buckeyes).

Pinnately compound leaves have the leaflets arranged along the main or mid-vein (called a rachis in this case).

Bipinnately compound leaves are twice divided: the leaflets are arranged along a secondary vein that is one of several branching off the rachis. Each leaflet is called a pinnule. The pinnules on one secondary vein are called pinna; e.g. Albizia (silk tree).

trifoliate: a pinnate leaf with just three leaflets, e.g. Trifolium (clover), Laburnum (laburnum).

pinnatifid: pinnately dissected to the midrib, but with the leaflets not entirely separate, e.g. some Sorbus (whitebeams).

Characteristics of the petiole:

Petiolated leaves have a petiole.

In peltate leaves, the petiole attaches to the blade inside from the blade margin.

Sessile or clasping leaves do not have a petiole. In sessile leaves the blade attaches directly to the stem. In clasping leaves, the blade partially or wholly surrounds the stem, giving the impression that the shoot grows through the leaf such as in Claytonia perfoliata of the purslane family (Portulacaceae).

In some Acacia species, such as the Koa Tree (Acacia koa), the petioles are expanded or broadened and function like leaf blades; these are called phyllodes. There may or may not be normal pinnate leaves at the tip of the phyllode.

Characteristics of the stipule

A stipule, present on the leaves of many dicotyledons, is an appendage on each side at the base of the petiole, resembling a small leaf. They may be lasting and not be shed (a stipulate leaf, such as in roses and beans); or be shed as the leaf expands, leaving a stipule scar on the twig (an exstipulate leaf).

The situation, arrangement, and structure of the stipules is called the stipulation.

There are two subtypes of venation, craspedodromus (the major veins stretch up to the margin of the leaf) and camptodromous (major veins come close to the margin, but bend before they get to it).

Feather-veined, reticulate — the veins arise pinnately from a single mid-vein and subdivide into veinlets. These, in turn, form a complicated network. This type of venation is typical for dicotyledons.

Pinnate-netted, penniribbed, penninerved, penniveined; the leaf has usually one main vein (called the mid-vein), with veinlets, smaller veins branching off laterally, usually somewhat parallel to each other; eg Malus (apples).

Three main veins originate from the base of the lamina, as in Ceanothus.

Palmate-netted, palmate-veined, fan-veined; several main veins diverge from near the leaf base where the petiole attaches, and radiate toward the edge of the leaf; e.g. most Acer (maples).

Parallel-veined, parallel-ribbed, parallel-nerved, penniparallel — veins run parallel most the length of the leaf, from the base to the apex. Commissural veins (small veins) connect the major parallel veins. Typical for most monocotyledons, such as grasses.

Dichotomous — There are no dominant bundles, with the veins forking regularly by pairs; found in Ginkgo and some pteridophytes.

See Also

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